1. Field of the Invention
The present invention relates to an equatorial telescope with a polar-axis telescope, and especially relates to a setting of the polar-axis telescope.
2. Description of the Related Art
In an equatorial telescope, a polar-axis telescope is firstly set so as to make the polar axis parallel to the rotation-axis of the earth. Namely, the optical axis of the polar-axis telescope, which is defined as a polar-axis, is set so as to be parallel to the rotation-axis by adjusting the position of the equatorial telescope mount. When observing a constellation in, for example, the Northern hemisphere, the Northern sky is aimed at by the polar-axis telescope. Then, the azimuth and altitude of the equatorial telescope mount are adjusted such that a specific target star, such as the polar star, appears at a proper position in the visual field of the polar-axis telescope.
After setting the polar-axis telescope, observed stars are aimed at by an astronomical telescope mounted on the equatorial telescope mount. The astronomical telescope is rotatable around the polar-axis, which enables the astronomical telescope to follow the target stars, with the passing of time, that is with the rotation of the earth. For example, a camera can be mounted on an eyepiece of the astronomical telescope, and a so called “bull photographing”, which requires a long exposure time, is performed.
When photographing a constellation, highly precise setting of the polar-axis is required to, for ex ample, take a high-quality photograph. However, when the magnification of the polar-axis telescope is set to a large magnitude, the visual field becomes narrow, so that the target star cannot be easily aimed at. On the other hand, when the magnification is set to a small magnitude, the precision of the polar-axis setting degrades.